204 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 195-4 



0.16 



0.14 



SI 



0.02 



1.0 

 CRoo 



Fig. 11 — Ampere turn sensitivity of a domed pole face electromagnet. 



rhere : 



(17) 



Q. = 



(RoCHi 



27ri?(R. 



(R„((Rg + (S^fY 1 + 27ri2(R„ * 



The ratio is a function of (Rf/(R„ and of /?(R^ , which determines 

 (Rg/(R„ > as shown by (14). Alternatively, 12 may be taken as a function 

 of the ratios (Rf/^(R« and 7?(3lp , and 9. may be represented, as in Fig. 11, 

 by a family of curves gi^^ing fi versus (Rf'/(R„ for various values of /?(R/? . 



From (17), maximum ampere turn sensitivity is attained by mini- 

 mizing the separation x and the reluctance (R^ external to the gap. 

 Assuming these to be made as small as engineering considerations per- 

 mit, the pull for a given value of A^/ varies as 12. With x and (51^ fixed, 

 12 now depends only on the dome radius R, to which E(^f is now pro- 

 portional, and on the projected area A, to which (Rf/(R«, is now propor- 

 tional. From the curves of Fig. 11 it is apparent that maximum ampere 

 turn sensitivity is attained by using as large a value of dome radius as 

 possible. For a given dome radius, there is an optimum value of (Rf/(R« , 



